1. Field of the Invention
The embodiments described herein are related to Ultra Wideband communication systems, and more particularly to efficient communication with reduced processing overhead in a Ultra Wideband communication system.
2. Background
FIG. 1 is a diagram illustrating an exemplary frame structure 102 used in Ultra Wide Band (UWB) communication systems. As can be seen, frame 102 comprises a preamble 104 separated from a packet payload, or data portion 108 by a header 106. Preamble 104 can comprise a packet sync sequence (SYNC) 110, a start frame delimiter (SFD) 112, and a channel estimation sequence (CE) 114. In many conventional systems, SYNC 110 is comprised of a plurality of codes 116. SYNC 110 is used to detect the beginning frame 102 and allows a device to lock to a specific piconet. Each piconet uses a different code 116 for SYNC 110. SFD 112 is used to indicate the end of SYNC 110 and CE 114 is used to estimate multi-path fading for the channel being received. SFD 112 also comprises of plurality of codes 118. CE 114 also comprises a plurality of codes 120.
Thus, in a UWB system a piconet could be configured to transmit data 108 within frame structures 102. UWB devices operating in range of the piconet can then be configured to receive packets 102 from the piconet and synchronize their receivers to the transmitter in the piconet by detecting and correlating SYNC 110 within frame 102. Because the device has no knowledge or where within SYNC 110 it begins receiving frame 102, SFD 112 can be included in order to indicate that the end of SYNC 110 is being received. Accordingly, when SFD 112 is detected by the receiver, the receiver can be configured to stop trying to synchronize itself with the transmitter and, if synchronization is achieved, prepare to receive data 108. CE 114 can be used by the receiver to estimate the effects of multi-path fading for the received signal.
The receiver front-end manipulates the received data and converts it to a digital signal that can be passed to a digital back-end within the receiver. This digital back-end can comprise the components necessary to determine, for example, whether the receiver is synchronized with the transmitter. FIG. 2 is a diagram illustrating an exemplary portion 200 of a digital back-end receiver configured to determine whether or not the receiver is synchronized with the transmitter in the piconet. Receiver portion 200 can comprise a code match filter 202, which can be configured to perform a coherent filtering, e.g., de-spreading, of the received signal. The coherent filtering, or dc-spreading process typically results in a processing gain being applied to the received signal at the output of code match filter 202.
The processing gain can be approximately equal to the number of chips included in code 116. Thus, for example, if code 116 comprises 127 chips, then a processing gain of 127 can be achieved by code match filter 202.
The output of code match filter 202 is then passed to a squarer 204, which can be configured to take the power of the signal at the output of code match filter 202 and perform a squaring operation. The output of squarer 204 can then be passed to accumulator 206, which can be configured in non-coherently accumulate the power of the output of square root 204.
The output of accumulator 206 can then be passed to a decision block 208, which can be configured to compare the accumulator output to a threshold and decide whether the piconet signal is in fact present. If the receiver is sufficiently synchronized with the piconet, then the power at the output of accumulator should be sufficient to surpass the threshold indicating that the piconet signal is in fact present. If signals from the piconet are not being received, or if the receiver is not synchronized with the transmitter in the piconet, then the power at the output of accumulator 206 should not be sufficient to surpass the threshold and decision block 208 should indicate that the piconet signal is not present.
FIG. 3 is a diagram illustrating an exemplary code match filter 202 that is often used in conventional receiver portions 200. Such a conventional code match filter 202 typically comprises a plurality of filter taps 302, e.g., 127 filter taps 302 for a code 116 comprising 127 chips. Filter taps 302 are separated by delay blocks 304 and are coupled with the inputs of multipliers 306, which can be configured to multiply the tap input by a coefficient, as is well known. The output of multipliers 306 can then be provided to an adder tree 308 where the outputs are combined into a single signal.
Such conventional code match filter designs suffer from several problems that can limit efficiency and over-all performance. For example, one challenge presented by such a conventional code match filter design is that the circuit has to run at a very high speed approximately equal to the bandwidth of the system. As is understood, UWB systems have extremely wide bandwidths, e.g., 1.5 GHz. Thus, code match filter 202 must be configured to run at a speed of approximately 1.5 GHz. This presents many design problems. One problem is that the circuits included in code match filter 202 can have difficulty running at such high speeds. Further, operation at such high speeds can present increase error tolerances, and consume much larger amounts of current. Another problem with such a conventional code match filter 202 is that it tends to be highly complex. For example, adder tree 308 requires many sub-adders, e.g. as many as 126-sub adders for a code 116 of length 127. This increased complexity increases error tolerances, increases the area required to implement the code match filter, and can also increase current consumption.
It will also be understood that at least in the United States, UWB systems are configured to operate in what is referred to as the unlicensed spectral band. Because many systems, including many government and safety systems, operate in the unlicensed band, the Federal Communications Commission (FCC) regulates the transmit power for devices operating in the unlicensed band. The regulations in some respects attempt to limit the peak output power. It will be understood that limiting the peak output power will also limit the transmit range of a device. It will also be understood that it is often preferable maximize the transmit range for effective and efficient communication. Accordingly, the FCC's regulations can be at odds with performance objectives.
It will be understood that it can be preferable to have a flat transmit spectrum. A flat transmit spectrum allows for the maximum transmit power even in the face of the FCC's regulations. It will also be understood that the above issues can limit the effectiveness and efficiency of conventional UWB receiver designs and prevent them from meeting the requirements of the specific implementation, e.g., in terms of battery life and size.